The present invention relates to a power detector and more particularly to a super heterodyned power detector with enhanced linearity for use in power leveling loops.
FIG. 1 shows a typical power leveling loop in which a source 10 provides a frequency Fl to power modulator 12. Modulator 12 provides an input to directional coupler 14, which in turn provides an output F1 on line 16 and an input F1 of broad band power detector 20. This design uses power detector 20 to convert the coupled signal to DC, which is then filtered through LPF 22 and compared to a power reference voltage 26 in difference amplifier 24. The difference signal 28 drives the loop integrator 29 which in turn drives the power modulator 12, so that the power is automatically adjusted until the detector voltage equals the reference voltage. This approach has several limitations:
1. The directional detector must operate at the measurement frequency. This degrades the performance in several ways. PA0 2. When the power is varied, the variation appears directly at the detector. Therefore, the power control accuracy is degraded by nonideality of the detector device. PA0 3. The directional coupler reduces the maximum power available to the test port.
a. The detector performance is not constant with frequency. Therefore, when the power is adjusted by changing the reference setting, the output power change is not constant with frequency. PA1 b. The detector must have enough bandwidth to operate at all output frequencies. For microwave systems, this can mean more than 10 gigahertz of bandwidth. This broad bandwidth limits the sensitivity of the leveling loop, since the noise power is proportional to bandwidth. As a result, the power adjustment range of the loop is reduced. In addition, since the detector responds to all frequencies within the bandwidth, output power at frequencies other than the desired output frequency will effect the power setting. PA1 c. The requirement for full frequency coverage can also result in added system complexity, since there may be no single detector with adequate bandwidth to cover the entire frequency range. If this is the case, some sort of frequency multiplexing with multiple detectors would become necessary.
In view of the foregoing, it would be desirable to provide an improved power detector.